DESCRIPTION: (Applicant's Description) The biochemical function of the BRCA1 tumor suppressor gene product is not understood. What little is known of how it operates suggests that it participates in the maintenance of genomic integrity. The protein is concentrated in nuclear foci (dots) in numerous cell lines and strains, and these structures also contain two, known BRCA1-associated proteins, BARD-1 and Rad 51. The function of the former is unknown; that of the latter is dedicated to proper strand exchange and certain other steps necessary for proper double strand break repair and homologous recombination. Nothing is known of why germ line BRCA1 mutations predispose affected individuals to the development of breast and/or ovarian cancer. An effort will be made to understand the biochemical significance of BRCA1 localization in nuclear dots; to search for and analyze the significance of its association with other BRCA1 binding proteins; to decipher the significance of the BRCA1/BARD1 interaction; to understand whether BRCA1 and Rad 51 interact directly or whether there are linker protein(s); and to analyze functional effects of Rad 51/BRCA1 complex formation.